The present invention relates to a plunger pump which solves the problem of a drop in pumping efficiency resulting from air or gas trapped in the sucked liquid staying in a compression chamber.
A prior art plunger pump is shown in FIG. 4 in which a plunger 1 has its one end projecting into a compression chamber 2 provided with an intake valve 3 and a discharge valve 4. The valves 3 and 4 have their respective ball-shaped valve bodies 3a and 4a which are biassed by springs 3b and 4b toward valve-closing positions with forces barely enough to keep the valves closed. In other words, the valve-opening pressure is set to a minimum.
Still, if the compression chamber 2 is filled with air or gas sucked in together with a liquid, it becomes difficult to increase and decrease sufficiently the pressure in the compression chamber to open the intake or discharge valve. In such a case, the pump efficiency may remarkably worsen or the pumping function may be totally lost. If the pump is provided with an accumulator in its discharge circuit, the gas such as nitrogen gas sealed in the accumulator might partially permeate through a partitioning rubber bladder to mix into the liquid. The leaked gas will find way through the discharge valve into the compression chamber and stick there. This may cause the accumulator to cease functioning.
With a side port type plunger pump in which an intake port is opened and closed according to the relative position of the plunger with respect to a pump case, the gas having flowed into the compression chamber is adapted to be carried back to a reservoir tank through an intake passage formed in the plunger. With this type of pump, in order to finish the sucking-up of liquid into the compression chamber within a limited time in its suction stroke, the intake passage has to have a sufficiently large sectional area. This results in a reduction in the effective stroke of the plunger, and thus in pumping power (or compression ratio).